Antarctic ice grows as climate warms

Feeling the heat

(Image: Seth Resnick/Science Faction/Corbis)

Call it a tale of two poles. While sea ice in the Arctic is vanishing fast, the extent of Antarctic ice has increased. Good explanations for the growth of ice in the Southern Ocean have been hard to find, but now the problem may have been cracked. Counter-intuitively, it seems global warming may be cooling southern surface waters.

Nobody predicted that the fate of ice at each pole would take such different paths in just 30 years, with Arctic sea ice dropping more than 15 per cent, even as Antarctic ice has risen by more than 5 per cent. The link between global warming and melting in the Arctic is clear cut, but the situation is more complex in the south. There, ocean water below a depth of 100 metres has been getting warmer, in line with rising ocean temperatures worldwide, but surface waters and the air above have become cooler.

The reason, say Richard Bintanja and colleagues at the Royal Netherlands Meteorological Institute, is that the deep warm water is melting the shelves of floating ice that extend from the continent. This is setting off flotillas of icebergs and creating a layer of cool, fresh water at the ocean’s surface. Measurements and previous modelling studies show this is happening, say Bintanja and his team.

They hypothesise that the layer of cool surface water insulates the remaining floating ice from warm deep currents. Using a climate model, they show that a realistic injection of cool meltwater should bulk up Antarctic ice. They predict these trends will continue.

It sounds like good news – a powerful negative feedback on global warming. And it is. But the process has an unexpected consequence for global sea levels.

Climate models predict that, in a warmer world, ice slipping off Antarctica will raise sea levels. But they also show warmer air will hold more moisture, generating more snowfall over Antarctica. Piled up on the continent, that snow will keep water out of the oceans and moderate or even reverse the sea level rise from Antarctic melting.

Bintanja says that is wrong. Warm deep ocean currents will keep eating away at the ice shelves. But the cooler than anticipated air will evaporate less moisture and produce less snow. “More water stays in the ocean,” he told New Scientist. Result&colon; a cooler climate but more sea level rise.

The issue needs to be resolved if we want accurate predictions of sea level rise. A draft of the next report by the Intergovernmental Panel on Climate Change could only say that the growth of Antarctic sea ice is “consistent with internal variability” – meaning it is essentially random. Testable hypotheses like those proposed by Bintanja and Holland help provide reliable forecasts.